1. Field of the Invention
The present invention relates to a magnetoresistive element that is for use in, for example, a thin-film magnetic head, and has a pair of free layers and a pair of side shields.
2. Description of the Related Art
Recently, magnetic disk drives have been improved in areal recording density, and thin-film magnetic heads of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head including an induction-type electromagnetic transducer for writing are stacked on a substrate.
Examples of MR elements include a giant magnetoresistive (GMR) element utilizing a giant magnetoresistive effect and a tunneling magnetoresistive (TMR) element utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, those incorporating spin-valve GMR elements or TMR elements have been mass-produced.
Spin-valve GMR elements and TMR elements each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer opposite from the spacer layer. The free layer is a ferromagnetic layer whose direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer whose direction of magnetization is pinned. The antiferromagnetic layer is a layer that pins the direction of magnetization of the pinned layer by means of exchange coupling with the pinned layer. For spin-valve GMR elements, the spacer layer is a nonmagnetic conductive layer. For TMR elements, the spacer layer is a tunnel barrier layer.
Examples of the read head incorporating a GMR element include one having a current-in-plane (CIP) structure in which a current used for detecting a signal magnetic field (hereinafter referred to as a sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element, and one having a current-perpendicular-to-plane (CPP) structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element.
The read head has a pair of shields with the MR element therebetween. The distance between the pair of shields is called a read gap length. Recently, with an increase in recording density, there have been increasing demands for a reduction in track width and a reduction in read gap length in read heads.
As one of MR elements that can achieve a reduction in read gap length, there has been proposed an MR element that includes a pair of ferromagnetic layers each functioning as a free layer, and a spacer layer disposed between the pair of ferromagnetic layers (such an MR element will hereinafter be referred to as an MR element of three-layer structure), as disclosed in U.S. Patent Application Publication No. 2009/0034132A1, U.S. Patent Application Publication No. 2009/0135529A1, and U.S. Patent Application Publication No. 2010/0027168A1, for example. In the MR element of three-layer structure, when the pair of ferromagnetic layers are subjected to no external magnetic field, they are magnetized in directions that are antiparallel to each other and parallel to the track width direction. The directions of magnetization of the pair of ferromagnetic layers change in response to an external magnetic field.
In the read head incorporating the MR element of three-layer structure, a bias magnetic field is applied to the pair of ferromagnetic layers. The bias magnetic field changes the directions of magnetization of the pair of ferromagnetic layers so that their directions of magnetization each form an angle of approximately 45 degrees with respect to the track width direction. This makes the relative angle between the directions of magnetization of the pair of ferromagnetic layers approximately 90 degrees. When a signal magnetic field sent from the recording medium is applied to the read head, the relative angle between the directions of magnetization of the pair of ferromagnetic layers changes, and consequently, the MR element changes in resistance. With such a read head, it is possible to detect the signal magnetic field by detecting the resistance of the MR element. The read head incorporating the MR element of three-layer structure allows a much greater reduction in read gap length as compared with a read head incorporating a conventional GMR element.
For the MR element of three-layer structure, one of methods for making the directions of magnetization of the pair of ferromagnetic layers antiparallel to each other when no external magnetic field is applied is to establish antiferromagnetic coupling between the pair of ferromagnetic layers via the spacer layer by the RKKY interaction.
Disadvantageously, however, this method imposes limitation on the material and thickness of the spacer layer so as to allow antiferromagnetic coupling between the pair of ferromagnetic layers. In addition, since this method limits the material of the spacer layer to nonmagnetic conductive materials, it is applicable to neither a TMR element which is expected to provide a high output, nor a GMR element of CPP structure of current-confined-path type which is an MR element that is also expected to provide a high output and whose spacer layer includes a current-transmitting portion and a current-blocking portion. The foregoing method further has the disadvantage that, even if it could be possible to make the directions of magnetization of the pair of ferromagnetic layers antiparallel to each other, it is difficult to make them parallel to the track width direction with reliability.
U.S. Patent Application Publication No. 2009/0135529A1 and U.S. Patent Application Publication No. 2010/0027168A1 each describe a method of controlling the directions of magnetization of a pair of free layers in an MR stack by using a pair of shields. The MR stack includes the pair of free layers and a spacer layer interposed between the pair of free layers. The pair of shields are provided so that the MR stack is interposed therebetween. According to this method, the pair of free layers in the MR stack are magnetically coupled to the pair of shields, and are thereby controlled so that their directions of magnetization are antiparallel to each other. Hereinafter, an MR element that is configured to include the foregoing MR stack and the pair of shields will be referred to as a shield-coupling MR element.
In order to increase the recording density, it is needed to reduce the read gap length and the track width as well. To reduce the track width, it is effective to reduce the dimension of the MR stack in the track width direction. However, there is a limit to the reduction of the dimension of the MR stack in the track width direction. It is therefore desired to introduce some technology that can reduce the effective track width for an identical dimension of the MR stack in the track width direction. One of such technologies is to provide a pair of side shield layers on opposite sides of the MR stack in the track width direction, as described in U.S. Patent Application Publication No. 2009/0034132 A1.
The inventors of this application then conceived providing a pair of side shield layers on opposite sides of the MR stack in the track width direction in a shield-coupling MR element, and prototyped such an MR element. As a result, it was found that the shield-coupling MR element having a pair of side shield layers can achieve reductions in read gap length and effective track width, and can thus provide improved recording density.
The configuration of the shield-coupling MR element with the pair of side shield layers described in U.S. Patent Application Publication No. 2009/0034132 A1, however, has room for improvement in the following respect. That is, each of the side shield layers has a predetermined dimension in the direction perpendicular to a medium facing surface, which is the surface facing the recording medium, of the thin-film magnetic head. This dimension is equal to, for example, the dimension of the MR stack in the direction perpendicular to the medium facing surface. Each of the side shield layers therefore has a relatively small volume. The pair of side shield layers described in U.S. Patent Application Publication No. 2009/0034132 A1 are magnetically isolated by insulating layers from a pair of shields that are arranged at the top and bottom of the MR stack, respectively. Thus, the side shield layers are relatively small in volume and are magnetically isolated. Consequently, such side shield layers can fail to provide a sufficient shielding function to absorb an excessive magnetic flux. In addition, the directions of magnetization of the side shield layers lack stability, which can make the MR element unstable in operation.